This invention relates to a method for predicting the equilibrated open-circuit voltage of an electrochemical energy store by measuring the voltage settling response Uo(t) in a load-free period.
For determining the state of charge of electrochemical energy stores, it is often possible to use its correlation with the open-circuit voltage. This applies to both primary and secondary stores (accumulators/batteries). One example of this is a lead-acid accumulator, in which the state of charge can be linked to the acid concentration, which can in turn be derived from the open-circuit voltage.
A difficulty which arises with many batteries systems and, in particular, with a lead-acid battery, is that the open-circuit voltage settles only very slowly after the battery has been loaded. The situation is often encountered that the rest pauses are so short that there is no opportunity to wait until the transients of the open-circuit voltage have settled to deduce the state of charge from its steady-state value.
DE 3520985 C2 describes a method for determining the state of charge of a lead-acid accumulator, which presupposes a substantially post-transient state of the lead-acid accumulator. For example, a resting time of at least five hours is recommended before the open-circuit voltage is assumed to have been reached and is determined, but the decay response is not evaluated.
DE-198 47 648 A1 and U.S. Pat. No. 6,163,133 describe a method for determining the state of charge of a lead-acid accumulator by measuring the open-circuit voltage, which uses the response of the voltage response before a post-transient state is reached. The approach selected there does not, however, take temperature dependency into account, which can cause interference especially at low temperatures, and it is restricted to evaluations at set times.
It would, therefore, be advantageous to be able to deduce the genuine open-circuit voltage from the time response of the transients of the load-free voltage (or the voltage under only a small load), before a steady-state open-circuit voltage has settled. This would provide the prerequisite for the use of a method to determine the state of charge from the open-circuit voltage value. It would also be advantageous to determine the actual equilibrated open-circuit voltage from the nonsteady-state voltage response of an unloaded electrochemical store, and the state of charge therefrom.
This invention relates to a method for predicting an equilibrated open-circuit voltage of an electrochemical storage battery including measuring a voltage settling response Uo(t) in a load-free period using a relationship (1) between an equilibrated open-circuit voltage Uoo and a decaying voltage Uo(t)
Uoo=Uo(t)xe2x88x92w*ln(t)xe2x88x92w*F(T)xe2x80x83xe2x80x83(1), 
wherein w is an experimentally determined slope of dependency of Uo on ln(t) at time t, w=xe2x88x92(Uo(t2)-Uo(t1))/ln(t2/t1), Uo(t1) is an unloaded voltage Uo at time t1, Uo(t2) is an unloaded voltage Uo at later time t2 greater than t1, and F(T) is a function which depends only on absolute temperature T of the battery.
This invention also relates to a method of predicting state of charge including measuring the voltage settling response of the storage battery and comparing the voltage settling response to true open-circuit voltage Uoo and the decay voltage Uo(t) to determine the state of charge.